American intelligence agencies estimate that nearly 43,000 fighters from more than 120 countries — including 250 Americans among 7,400 Westerners — have gone or tried to go to Syria and Iraq since 2011.
While Turkey’s border tightening and other intelligence and law enforcement measures have by some estimates cut in half the monthly flow into Syria and Iraq, American analysts say as many as 500 to 1,000 fighters a month are still pouring in, with hundreds of others heeding the Islamic State’s call to go to affiliates in Libya or Afghanistan instead, or remain at home and carry out attacks from there.
Earlier this month, a top United Nations official said that nearly 30,000 of those foreign fighters remained in Syria and Iraq — far more than Western intelligence agencies had estimated. The official, Jean-Paul Laborde, a United Nations assistant secretary general and head of its counterterrorism committee, told reporters in Geneva that as the Islamic State loses territory in Iraq and Syria, “we are seeing them return, not only to Europe but to all of their countries of origin, like Tunisia, Morocco.”
American military and intelligence analysts are combing through the documents and electronic data recovered in Manbij, hoping to add to their growing knowledge of the rosters of Islamic State fighters and to help identify, locate and attack fighters in Syria and Iraq.
In a speech Wednesday at Fort Bragg, N.C., Mr. Carter described Manbij as “a key transit point for external plotters threatening our homelands. And there we’re already beginning to gain and exploit intelligence that’s helping us map their networks of foreign fighters.”
Another use of the documents is, as Mr. McGurk said, to alert foreign intelligence and counterterrorism services across Europe, the Mideast and North Africa, even as a spate of terrorist attacks in France and Germany — some apparently inspired by the Islamic State — have roiled Europe.
Any information from the Manbij trove would augment the activities of a sensitive intelligence-coordination center at a military base in Jordan called Operation Gallant Phoenix.
At the base, military, counterterrorism and law enforcement agencies from several countries use publicly available software to sift through open-source information. The Pentagon-led effort has caused turf war tensions with the C.I.A. in Jordan, but supporters of the program have prevailed, sending names and other leads back to foreign capitals for investigation.
The latest trove of documents was collected in various locations in the region around Manbij, where Syrian Kurdish and Arab fighters, backed by American Special Operations commandos, have battled Islamic State fighters at a crucial junction between the Turkish border and Raqqa, the Islamic State’s de facto capital in Syria.
“The operation in Manbij is about shutting down the main corridor from Raqqa and then out, in which some of the attackers that launched the Paris attacks we know traveled through that route,” Mr. McGurk said, referring to the Islamic State’s assault on Paris in November. “By shutting that down, you make it harder for them to kind of plan the larger-scale, kind of more coordinated attacks.”
The Syrian Observatory for Human Rights, based in Britain, has reported that more than 100 civilians have died in airstrikes in and around Manbij since late May, when the American-backed militias started their offensive against Islamic State fighters there.
The documents recovered in Manbij recall an American commando raid in the summer of 2007 on a suspected Qaeda safe house in the Iraqi town of Sinjar, near the Syrian border. That assault yielded documents containing information about Syrian smuggling networks used to move foreign fighters into Iraq to fight for Al Qaeda. The most significant discovery was a collection of biographical sketches that listed hometowns, dates of birth, aliases and other details for more than 700 fighters brought into Iraq since August 2006. American officials later used the information to pressure the fighters’ home countries to crack down on the flow.
American officials express confidence that the latest cache will yield similar insights.
“We are learning more about Daesh at all levels from this,” said Col. Christopher Garver, a spokesman for the United States military in Iraq, using an Arabic acronym for the Islamic State.
“We’ve learned about how they organize their governance structures to ensure they can completely control all aspects of daily life, from religious practice, to education to tax collection and management of central services.”
“We have a better understanding of how Daesh facilitates foreign fighter movements into and out of Syria and Iraq, which gives us valuable insight into stopping the flow of foreign fighters into the region,” Colonel Garver said.